According to the HLS+WebVTT spec, if there is no text in a segment
a webvtt file with no cues can be added to the manifest. By outputting
empty segments it allows the accumulated duration in the Master Playlist
to better represent the duration of the text stream.
Spec Reference: //tools.ietf.org/html/draft-pantos-http-live-streaming-23
Section 3.5. WebVTT
Bug: #205
Change-Id: I5de01200fd9fa99c57949c773e8ee926b0f6ba8a
Connected the WebVtt Pipeline together to test that the parser,
segmenter, and output all work together as expected.
Change-Id: I07138f7b5b1318f84c27c5b607d8df207c57ddb3
Aaron Vaage